1. Field of the Invention
The invention relates to a device for optically reading an optical recording medium applicable more particularly to the optical heads for reading optical disks designated in the trade by expressions such as compact disks, video disks or digital optical disks.
2. Description of the Prior Art
An optical disk reading device of a known type includes at least, as shown in FIG. 1:
a laser source emitting a linearly polarized light beam 1; PA0 a polarization separator element 8; PA0 a quarter wave plate 3; PA0 a focusing objective 4; PA0 and an assembly of signal and positioning error detectors 9. PA0 a laser source emitting, along a given axis, a first linearly polarized beam and of given wave length; PA0 a separator receiving the first beam and retransmitting it in the form of a second beam: PA0 a focusing lens which focuses this second beam in the form of a fourth reading beam; PA0 a recording medium receiving the fourth reading beam and reflecting it in the form of a fifth beam to the focusing lens, and the beam separator which retransmits at least one detection beam; PA0 at least one detection device receiving the detection beam and delivering at least one electric signal representing the detection performed; PA0 wherein the beam separator includes a phase network including means inducing an effect of astigmatism in the detection beam. PA0 a laser source emitting, along a given axis, a first linearly polarized beam of given wave length; PA0 a polarization beam separator receiving the first beam and retransmitting it in the form of a second beam to a quarter wave plate which in exchange transmits a third circularly polarized beam; PA0 a focusing lens which focuses this third beam in the form of a fourth reading beam; PA0 a recording medium receiving the fourth reading beam and reflecting it in the form of a fifth beam to the focusing lens, the quarter wave plate and the polarization beam separator which retransmits at least one detection beam; PA0 at least one detection device receiving the detection beam and delivering at least one electric signal representing the detection performed; PA0 wherein the polarization beam separator includes a phase network between a first medium and a second medium at least one of which is birefringent, the indices of the two media being equal for the linear polarization of the first beam.
In the prior art, the polarization separator element is a glass cube having in its median plane a superimposition of thin dielectric layers 82 reflecting a tangentially polarized wave and transmitting a sagittally polarized wave.
The laser source 1 emits a light wave whose polarization is such that the beam is reflected, by the polarization separator, to the quarter wave plate 3. The beam is then transformed into a circularly polarized beam and is focused by lens 4 on disk 5. Depending on the information met on disk 5, the beam is reflected with a phase shift. The quarter wave plate 3 then transforms the circularly polarized beam into a linearly polarized light beam. The beam obtained is retransmitted by the separator cube 8 to the detector assembly 9 which may, in FIG. 1, be a photodetector. The amount of light detected by the photodetector depends on the size of the phase shift introduced by disk 5 and so on the amount of light retransmitted by the separator cube 8.
Such a system has different drawbacks. First of all, the positioning of the detector assembly 9 with respect to source 1 and disk 5 through the separator cube 8 is difficult to achieve. Such positioning involves tolerances of a few microns as regards the position of the separator cube.
In addition, such a separator cube is costly.
Finally, the assembly thus described is relatively space consuming.
The invention overcomes these three types of drawbacks by providing an inexpensive optical reading device which is compact and whose design involves no constraints as regards mounting and positioning of the elements with respect to each other.